The invention relates to polarization mode dispersion and, more particularly, to the emulation of and compensation for such dispersion in optical fibers.
Tremendous quantities of information are transmitted as pulses of light over long distances through optical fibers. Ideally, the light pulses employed to encode the information for transmission would be as brief as light-modulating technology would permit. Using such short-duration pulses would enable the transmission of data at rates substantially greater than any rates currently available. But, among other impediments, distortion of the light pulses severely limits the rate at which the pulses may be intelligibly transmitted. That is, a variety of pulse distortion mechanisms increase the bit error rate (BER) to unacceptable levels at much lower transmission rates than would be dictated by the rapidity of light-modulating capabilities alone. Modal dispersion, for example, can severely distort pulses as they travel along an optical fiber. However, chromatic dispersion can be substantially eliminated through the use of single-mode optical fibers. Another form of pulse distortion, one which significantly contributes to the limitations of light-based information transmission and which is substantially unaffected by the use of single-mode fiber, is that of polarization mode dispersion (PMD). PMD is discussed, for example, in W. Eickhoff, Y. Yen and R. Ulrich, Applied Optics, Vol 20, 3428, 1981, which is hereby incorporated by reference. PMD, briefly, causes a frequency dependent variation in the polarization state of light injected into a fiber, resulting in pulse distortion. All optical fibers that are suitable for optical information transmission, including single mode fibers, exhibit PMD to a greater or lesser degree. Consequently, PMD may be the limiting factor for the transmission of data through any optical fiber.
Random changes in the birefringence of an optical fiber that occur along the light path cause PMD phenomena. These phenomena lead to the distortion of light pulses which travel along the light path and penalize the operation of fiber-based systems, particularly for high bit-rate operations. The PMD of a fiber is commonly characterized by two specific orthogonal states of polarization, called the principal states of polarization (PSP), and the differential group delay (DGD) between the PSPs. Typical DGD values are on the order of 1 to 100 psec.
First order PMD compensators are known and discussed, for example, in F. Heismann et al, ECOC 98 p. 529-530, Madrid 98, and D. Schlump et al, ECOC98, pp. 535-53, Madrid 98, which are hereby incorporated by reference in their entirety. Nevertheless, because the bit-rate of single channel transmission systems has reached the 40 Gb/s region, the impact of higher order PMD may become a limiting factor in fiber-optic data transmission. PMD compensators suitable for higher order PMD compensation have been discussed by J. Patscher et al, Electronic Letters, 1997, 33, pp.1157-9, and R. Noe et al, ECOC 98, post deadline, pp157-8, Madrid 98, which are hereby incorporated by reference,
Theoretical and experimental studies of effects caused by 2nd order PMD have been reported by C. Francia et al, ECOC 98 pp.143-144, Madrid 1998 and C Francia et al, IEEE PTL, pp. 1739-1741, Dc. 1998, which are hereby incorporated by reference. Methods for 1st and 2nd order PMD measurement have also been demonstrated, for example, by L. Nelson et al, submitted to ECOC 99, Nice 1999, which is hereby incorporated by reference. However, determining the influence of 2nd order PMD on a signal or evaluating the performance of 2nd order PMD compensators can not easily be done due to the fact that installed fibers have in general PMD of higher than 2nd order, and the known methods for emulating 2nd PMD have the disadvantage of generating simultaneously uncontrollable higher order PMD effects.
In order to accurately characterize the data transmission capacity of an optical fiber and an associated data transmission system it would be highly desirable to emulate the PMD of a fiber. Additionally, to improve the performance of such a fiber and its associated data transmission system, it would be highly desirable to compensate for such PMD.
In accordance with the principles of the present invention, an emulator may limit PMD generation to 1st and 2nd order (Where the definition of 1st and 2nd order conform to that of C. Francia et al, ECOC 98 pp. 143-144, Madrid 1998 and C Francia et al, IEEE PTL, pp. 1739-1741, Dc. 1998, previously discussed). Such an embodiment may be employed for PMD compensator testing as well as studying the impact of third and higher order PMD effects. By emulating 1st and 2nd order PMD and making a comparison between the transmitted signals over installed fibers and the adjusted emulator with same PMD parameters for 1st and 2nd order, unambiguous studies of higher order PMD effects can be conducted. Additionally a 2nd order PMD compensator in accordance with the principles of the present invention may be employed to compensate for higher order PMD, thereby improving the data transmission capabilities of optical fibers.
A polarization mode dispersion (PMD) emulation apparatus in accordance with the principles of the present invention may include one or more modular xe2x80x9ccellsxe2x80x9d that emulate PMD. In an illustrative embodiment each of the cells include optical delay and phase modulation components. The optical delay and/or the phase modulation components may be adjusted to account for differences in PMD and two or more of the cells may be combined to further adjust the overall PMD of the apparatus.
A PMD compensation apparatus in accordance with the principles of the present invention may include one or more modular xe2x80x9ccellsxe2x80x9d that compensate for PMD. In an illustrative embodiment each of the cells include optical delay and phase modulation components. The optical delay and/or the phase modulation components may be adjusted to compensate for various PMD values and two or more of the cells may be combined to further adjust the overall PMD compensation of the apparatus. In another aspect of the invention, the compensation apparatus may be used to compensate for PMD in wideband applications, such as wavelength division multiplexed (WDM) systems.